1. Field of the Invention
The present invention relates to a process for producing a porous fuel absorbent to be used in evaporation loss control devices and the porous fuel absorbent produced by the process.
2. Description of the Related Arts
A fairly large amount of fuel evaporates when it is supplied to an automobile fuel tank through a feed nozzle. Part of the fuel in the fuel tank and the carburetor's float chamber also evaporates while the automobile is running or at rest.
In order to prevent the leakage of the evaporated fuel into the air atmosphere, a canister filled with an absorbent is attached to, e.g., the fuel tank. The absorbent in the canister serves to trap the evaporated fuel. Such an absorbent filled evaporation loss control device is also used to trap fuel evaporating or leaking from fuel storage tanks other than those installed on automobiles. In such evaporation loss control devices, charcoal has hitherto been used as a fuel absorbent. Fuel absorbed by charcoal can be desorbed therefrom at the time when the devices are subjected to purging. Charcoal can therefore be used repeatedly with repeated absorption and desorption of fuel (see FIG. 7 and explanations therefore given hereinbelow). However, canisters employing charcoal often fall into a state where the devices are incapable of trapping evaporated fuel to a sufficient degree and hence the untrapped fuel vapor is released into the air atmosphere.
The present inventors have investigated the cause of the phenomenon. As a result, it has now been found that the working capacity of charcoal (i.e., the capability of charcoal to trap gasoline vapor) decreases significantly when it is in contact with liquid gasoline. It has also been found that such a contact between charcoal and liquid gasoline occurs when liquid gasoline condensed on the inner surfaces of the piping around the canister or on the charcoal-free space at the top of the canister comes into contact with the charcoal.
Such undesirable condensation of gasoline vapor in the piping and the canister occurs most frequently when the temperature of the ambient air is high or when the vapor pressure of gasoline is very high in the fuel tank or carburetor.
There is another factor that causes the undesirable decrease in the working capacity of charcoal. While the fuel vapor absorbed on the charcoal is composed of various molecules of different sizes, those having not more than 4 or 5 carbon atoms can be readily desorbed during the canister purging step, but larger molecules could not be desorbed as readily as the small ones. Because of this difference in desorption rate, the working capacity of charcoal decreases as it is used for a long period of time.
It has been proposed to use organic polymers (such as polypropylenes, styrene-butadiene copolymers, etc.) in place of charcoal (see Japanese Patent Application (Laid Open) Nos. 67,222/89 and 227,861/89). The fuel-absorbing ability of such an absorbent decreases when the absorption and desorption of fuel (or absorption-desorption cycle) is repeated. This is presumably because the strength of primary particles and the bonding force between primary particles (or strength of secondary particles) are low, and hence primary particles are destroyed and secondary particles are disintegrated when they are repeatedly subjected, e.g., to swelling (during absorption) and shrinking (during desorption), or to vibration. Such destruction and disintegration lead to a lowering in its absorption capability due to the generation of fine particles, which easily scatter and cause an uneven distribution of the adsorbent. At the same time, with the decrease in the particle size, the percentage of voids (or porosity) in the absorbent becomes smaller. This also leads to a decrease in absorption capability since its pores become more liable to clog during swelling.